Study design and vaccination

These retrospective study included a total of 2630 elderly people aged ≥60 years (mean age 80, range 60–104) enrolled in 21 seasons. After providing informed consent, all the subjects received 1 dose of trivalent inactivated commercially available influenza vaccine intramuscularly, in the deltoid or intradermally. Each dose of vaccine consisted of 15 µg of HA in a 0.5 ml dose (for vaccines administered intramuscularly) or in a 0.1 ml dose (for vaccines administered intradermally) of each of the three influenza strain antigens (A/H3N2, A/H1N1 and B influenza viruses). At time for recruitment of this study, demographic data, health status (evaluated using the data collected using the Italian VAOR (Valutazione Anziano Ospite di Residenza) schedule) and history of influenza vaccination of the preceding year were obtained for each subjects. Serum samples were obtained from the same subject before and 1 month after vaccination. Subjects were included in this study if they did not have a history of immediate hypersensitivity to eggs components. Subjects suffering from specific illnesses or chronic condition were not excluded. The study was conducted in Italy according to the Declaration of Helsinki and Good Clinical Practices. Since vaccines were assigned by local Health Authorities within the annual influenza campaign and sera were leftover sera from samples collected for clinical routine controls, the study did not need to be registered as a formal trial.